Press muffles for dental technology usually consist of ceramic embedding masses containing cristobalite, which are cast around a positive model of a dental restoration part. The dental restoration part or parts is/are preferably included in the muffle in a central position. The muffle is heated after having cured, such that the wax components completely disappear and shaping spaces for the dental restoration parts remain which can be filled with dental ceramics via a press channel.
For cost reasons, it is desirable to be able to press a number of dental restoration parts at a time, for which purpose the press channel is branched in a fashion similar to a tree and individual shaping spaces are connected with the press channel via connecting channels.
Due to the provision of multiple shaping spaces, the distance between the outer circumference and the respective shaping space differs. To maintain the quality of the dental restoration part, however, it is critical that a precise temperature profile is observed during the pressing process.
When putting into practice embedding masses for enameling furnaces, i.e., not pressing furnaces, it has become known from DE 38 44 151 to provide the embedding mass with chrome particles or tungsten particles in order to avoid any heat accumulation. This happens in immediate proximity to the dental restoration parts, in such a fashion that the chrome particles are in contact with the prime components there. However, this may easily lead to oxidation and to a discoloration of the prime components, which is unacceptable especially when the dental restoration part is used in the frontal region.
In order to equalize a temperature profile inside the muffle, it has been suggested furthermore to use a metal ring extending at the outer circumference of the muffle. This is supposed to have a supportive effect at the same time and to prevent the muffle from chipping or spalling during the pressing process. On the other hand, metal typically has a comparatively large coefficient of thermal expansion, such that the desired supportive effect is not provided during pressing to the extent desired.
Although temperature equalization can also be achieved without any problem by a corresponding waiting time, such a cycle time can then easily mean 8 hours, i.e., one entire work day, in case of large muffles with multiple shaping spaces, such as, for instance, 10-member bridges, which is not desired either.